Nanomaterials possess unique properties such as the quantum confinement effect, large surface-to-volume atom ratio, enhanced catalytic activities, tunable plasmonic responses to light, and more recently demonstrated property of enhanced absorption of X-rays and increased energy deposition density from X-ray absorption (Zhou H S et al., Phys. Rev. B 1994, 50, 12052-12056; Benfield R E., J. Chem. Soc. Faraday T. 1992, 88, 1107-1110; Jones G et al., J. Catal. 2008, 259, 147-160; Qu Y et al., J. Phys. Chem. Lett. 2010, 1, 254-259; Freunscht P et al., Chem. Phys. Lett. 1997, 281, 372-378; Alexander K et al., Nano Lett. 2010, 10, 4488-4493; Carter J D et al., J. Phys. Chem. B 2007, 111, 11622-11625; Lee C et al., J. Phys. Chem. C 2012, 116, 11292-11297; Cho S et al., Med. Phys. 2010, 37, 3809-3816). Chemical enhancement of X-ray effect by nanomaterials under X-ray irradiation has been demonstrated recently using gold nanoparticles under X-ray irradiation (Cheng N N et al., J. Am. Chem. Soc. Commun. 2012, 134, 1950-1953). The discovery of chemical enhancement by nanostructures as a result of the interaction of nanomaterials with X-ray generated reactive oxygen species (ROS) may greatly improve the ability of X-rays to form or break chemical bonds. This new property may impact many new applications such as activation of nano scale reaction mechanisms in the body, causing explosions in enclosed volumes, charging batteries at high rates, or converting ionizing radiation energy to chemical energy. This new research area is called X-ray Nanochemistry.
Several examples of chemical enhancement have been demonstrated in the literature, which include enhanced yield of DNA cleavage and reactions to form fluorescent molecules (Carter J D et al., J. Phys. Chem. B 2007, 111, 11622-11625; Foley E et al., Chem. Commun. 2005, 3192-3194; Carter J D et al., J. Colloid Interf. Sci. 2012, 378, 70-76). Since polymerization may be initiated using various means including X-rays, it is natural to consider nanomaterials may enhance such a process. It would be a significant scientific and technological advancement if a small dose of X-rays can trigger the growth of polymers, especially at remote and not easily accessible locations by light, or in an environment difficult to deliver large amounts of chemical initiators. One such place is tissue in the body (Tseng S J et al., Soft Matter 2012, 8, 1420-1427). Other potential applications include X-ray nanoscale lithography (Gaeta C J et al., J. Vac. Sci. & Technol. B. 2003, 21, 280-287; Toyota E et al., J. Vac. Sci. & Technol. B. 2001, 19, 2428-2433).
X-ray induced polymerization in the bulk has been widely studied (Collinson E et al., T. Faraday Soc, 1957, 53, 476-488; Kuzyk M G et al., J. Polym. Sci. Poly. Phys. 1988, 26, 277-287; Cleland M R et al., Rad. Phys. Chem. 2009, 78, 535-538). For example, Lee et al. and Blinova et al. studied formation of nanoparticles from Ag ions under X-ray irradiation (Blinova N V et al., Polymer 2009, 50, 50-56; Lee K P et al., Compos. Sci. Technol. 2007, 67, 811-816), a process investigated broadly by many groups in the last two decades (Belloni J et al., Radiation Chemistry: Present Status and Future Trends; 1st ed.; Jonah, C. D., Rao, B. S. M., Eds.; Elsevier Science, 2001; Vol. 87; pp 411-452). Merga et al. used p-aminothiolphenol (ATP) to probe the effect of X-rays on SERS substrate of gold nanoparticles (AuNPs) (Merga G et al., J. Phys. Chem. C. 2010, 114, 14811-14818). In another study, Felix et al. used X-rays to produce polyaniline (PANI) in aqueous solution (Felix J F et al., Synthetic Metals 2011, 161, 173-176). However, no conclusions have been drawn as to whether nanomaterials under X-rays enhance polymerization, partially due to (1) strong spontaneous growth of polymers in the presence of these nanomaterials such as AuNPs even without X-rays and (2) low sensitivity of SERS by AuNPs.
If polymer formation is enhanced by nanomaterials, then the growth process should occur on the surface of nanomaterials and the polymers so grown should be surface bound. SERS hence should be an ideal tool to probe polymer formation on the surface, as long as the polymers have intense SERS spectra. PANI is a good system for several reasons. First, it is SERS active. Second, PANI is an important polymer because it is one of a few conductive polymers (Cristescu C et al., J. Optoelectron. Adv. M. 2008, 10, 2985-2987). For example, PANI memory systems have been studied by Tseng et al (Tseng R J et al., Appl. Phys. Lett. 2007, 90). Third, PANI formation was investigated using SERS by Gao el al (Gao P et al., J. Phys. Chem. 1989, 93, 3753-3760), and its complex SERS spectra carry rich information of the type of PANI and SERS substrate. Baibarac et al. and Dong et al. investigated different forms of PANI deposited on different SERS substrates (Baibarac M et al., Synthetic Met. 1998, 96, 63-70; Dong X et al., Spectrochim. Acta A 2012, 88, 97-101). Theoretical studies such as DFT calculations were performed by Qi et al. (Qi Y et al., J. Raman Spectrosc. 2011, 42, 1287-1293). These investigations provide the knowledge basis for data analysis and mechanistic explanations even without X-rays and low sensitivity of SERS by AuNPs.